Spatial Light Modulators (SLMs) based on electromechanical grating devices have been adapted to a wide range of applications, including display, data storage, spectroscopy and printing. Such systems typically use large numbers of individually addressable devices in either a linear array or an area array, with over a million addressable devices desirable for an area modulator array in a high-quality display.
Linear arrays are particularly advantaged over their area array counterparts by virtue of higher resolution, reduced cost, and simplified optics. One advantage of particular interest: linear arrays are more suitable modulators for laser light than are many of their two-dimensional counterparts, such as liquid crystal-based modulators. Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360 (Bloom et al.) are one earlier type of linear array, for example, that offers a workable solution for high-brightness imaging, using laser sources. Another experimental type of linear array just recently disclosed and in early development stages is the flexible micromirror linear array, as described in the article “Flexible micromirror linear array for high resolution projection display” by Francis Picard, et al. in MOEMS Display and Imaging Systems, Proceedings of SPIE Vol. 4985 (2003). The prototype flexible micromirror linear array described in the Picard et al. article employs a line of reflective “microbridges” which are individually switched to modulate light to form a linear image.
Recently, an electromechanical conformal grating device including ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued on Oct. 23, 2001, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.” The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device or, more simply, the GEMS device, with ‘GEMS’ standing for ‘grating electromechanical system’. The GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This angular orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system, and enables a simpler optical system design with smaller optical elements.
Display systems based on a linear array of GEMS devices are disclosed by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,” issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,476,848, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SEGMENTED WAVEPLATE,” issued Nov. 5, 2002. Display systems based on GLV devices are disclosed in U.S. Pat. No. 5,982,553, entitled “DISPLAY DEVICE INCORPORATING ONE-DIMENSIONAL GRATING LIGHT-VALVE ARRAY” issued to Bloom et al. on Nov. 9, 1999.
Current color display system architectures for electromechanical grating devices of both GLV and GEMS types generally employ three separate color paths, Red, Green, and Blue (RGB), wherein each color path is provided with a linear array of electromechanical grating devices. Each linear array of electromechanical grating devices modulates its corresponding component red, green, or blue laser light. The resulting modulated light beams are then combined onto the same output axis. A full-color image is formed by scanning the modulated light beams across a display screen. Referring to FIG. 1, there is shown, in simplified block diagram form, a prior art display system 10 using three separate optical paths for a GEMS device. (An embodiment using a GLV device would also employ three color paths, but require a more complex arrangement of components.)
For red color modulation in a red modulation assembly 120r, a red light source 70r, typically a laser, provides illumination that is conditioned through a spherical lens 72r and a cylindrical lens 74r and directed towards a turning mirror 82r. Light reflected from turning mirror 82r is modulated by diffraction at an electromechanical grating light modulator 85r. Modulated diffracted light from electromechanical grating light modulator 85r is diffracted past turning mirror 82r and to a color combiner 100, such as an X-cube or other dichroic combiner. The modulated light from color combiner 100 is then directed by a lens 75, through an optional cross-order filter 110 (not shown), to a scanning mirror 77 for projection onto a display surface 90. Green color modulation in a green modulation assembly 120g uses a similar set of components for providing light to color combiner 100, with a green light source 70g, typically a laser, providing illumination through a spherical lens 72g and a cylindrical lens 74g and directed towards a turning mirror 82g. Light reflected from turning mirror 82g is modulated by diffraction at an electromechanical grating light modulator 85g. Modulated diffracted light from electromechanical grating light modulator 85g is diffracted past turning mirror 82g and to color combiner 100. Similarly, in a blue modulation assembly 120b, blue light source 70b, typically a laser, provides illumination through a spherical lens 72b and a cylindrical lens 74b and directs light towards a turning mirror 82b. Light reflected from turning mirror 82b is modulated by diffraction at an electromechanical grating light modulator 85b, diffracted past turning mirror 82b and to color combiner 100. In each color channel, turning mirror 82r, 82g, or 82b, acts as an obstructing element for blocking the reflected zeroeth order light from its respective electromechanical grating light modulator 85r, 85g, or 85b. 
The arrangement of components of FIG. 1 has been shown to provide full-color images having high spatial resolution, with excellent bit depth, high brightness, good contrast, and a broad color gamut when light sources 70r, 70g, and 70b are lasers. However, one problem that is common to systems using electromechanical grating light modulators with lasers or, more generally, used with highly coherent light sources, is speckle. Speckle is quantified in terms of contrast, C, given in percent as:
  C  =      100    *          (                        I          std                          I          mean                    )      wherein Istd is a standard deviation of intensity fluctuation about a mean intensity Imean. The speckle contrast for fully developed speckle is 100%. A result of perceived interference effects from scattering of the illuminating laser light by surface or volume scatterers of the projection screen, speckle reduces the ability of an imaging system to resolve fine spatial detail and causes levels of noise in an image that can be highly visually annoying. At worst, without some form of correction, speckle can be sufficiently objectionable to make coherent illumination unsuitable for display purposes. As a rule-of-thumb, visually pleasing images should have a speckle contrast of less than about 10%.
There have been a number of methods employed for reducing speckle effects in imaging displays. Conventional strategies for speckle reduction include modifying the spatial or temporal coherence of the illumination or modifying its polarization state. One method provides vibration or oscillatory movement of the display screen. With oscillation above a threshold speed, perceived speckle can be significantly reduced. Other methods include broadening the spectral line width of the laser illumination and reducing the spatial coherence by using static and oscillating diffusers or oscillating fibers or by vibrating various optical components in the path of illumination or imaging light.
Examples of proposed solutions that could be adapted for limiting speckle in systems employing electromechanical grating devices include the following:                U.S. Pat. No. 6,747,781 entitled “METHOD, APPARATUS, AND DIFFUSER FOR REDUCING LASER SPECKLE” to Trisnadi et al. discloses moving a diffusing element that is positioned at an intermediate image plane that subdivides image pixels into smaller cells having different temporal phase;        Commonly assigned U.S. Pat. No. 6,577,429 entitled “LASER PROJECTION DISPLAY SYSTEM” to Kurtz et al. discloses using an electronically controllable despeckling modulator to provide controllable, locally randomized phase changes with a linear SLM;        U.S. Pat. No. 6,323,984 entitled “METHOD AND APPARATUS FOR REDUCING LASER SPECKLE” to Trisnadi et al. discloses speckle reduction using a wavefront modulator in the image plane;        U.S. Pat. No. 5,313,479 entitled “SPECKLE-FREE DISPLAY SYSTEM USING COHERENT LIGHT” to Florence discloses illumination of a light valve through a rotating diffuser; and,        U.S. Pat. No. 4,256,363 entitled “SPECKLE SUPPRESSION OF HOLOGRAPHIC MICROSCOPY” to Briones and U.S. Pat. No. 4,143,943 entitled “REAR PROJECTION SCREEN SYSTEM” to Rawson disclose apparatus that reduce speckle by moving diffusive components in the projection path.        
While conventional methods for speckle reduction may have some applicability to laser projection systems using GEMS and other types of electromechanical grating devices, there are drawbacks to these approaches that constrain image quality and reduce overall contrast as well as adding cost and complexity to projection apparatus. Speckle remains a problem, therefore, that is only mitigated to some degree using conventional procedures. Thus, it can be seen that there would be benefits to a display apparatus employing electromechanical grating device technology that provides reduced speckle without the addition of separate diffusing or polarizing components.